There’s more carbon locked up in the high north’s permafrost than the combined total released into the atmosphere by humans. The big melt going on right now in the Arctic could trigger a train of catastrophic events across the world.

The Arctic’s soil and permafrost hold nearly twice as much carbon as the earth’s atmosphere, dwarfing the amount of carbon emitted to date by burning fossil fuels. Since the industrial revolution our dependence on coal and oil has ratcheted up the atmosphere’s carbon content, from 560 to 760 gigatons. Permafrost holds an estimated 1,400 gigatons of carbon.

In addition to carbon dioxide, the frozen source is releasing methane, a greenhouse gas 25 times more potent, though it stays in the atmosphere for only a decade rather than for millennia. The gas is bubbling up from land and also from a large, previously overlooked source: permafrost submerged beneath the Arctic Ocean. It’s too early to say whether this is a newly observed steady leak, or if it signals the beginning of a flood of methane.